Method for precise conversion of siderite into magnetite by neutral roasting

Abstract

The application discloses a method for accurately converting sphaerite into magnetite through neutral roasting, and belongs to the technical field of mineral processing. The method places sphaerite in a nitrogen atmosphere for neutral roasting to obtain an intermediate product containing ferrous oxide; then, through valve pulse opening and closing operation, the ferrous oxide particles are activated through pressure oscillation of carbon dioxide released by the decomposition of sphaerite itself; after nitrogen blowing, mixed gas of nitrogen and air is introduced into the device in stages, and the number of aeration times is determined according to the ratio of ferrous oxide to total iron in the intermediate product, so that a roasting product with magnetite as the main iron phase is finally obtained. The application activates the ferrous oxide particles through pressure oscillation generated by the valve pulse opening and closing, and combines with the stage type micro-oxygen accurate control, so that efficient conversion of sphaerite into magnetite is realized, the over-oxidation to generate hematite is effectively avoided, the roasting product has strong magnetism, and is convenient for subsequent magnetic separation and recovery.

Description

Technical Field

[0001] This invention belongs to the field of mineral processing technology, specifically relating to a method for the precise conversion of siderite into magnetite by neutral roasting. Background Technology

[0002] Siderite is an important low-grade complex iron ore resource in my country. Its theoretical iron grade is only 48.2%, and during geological formation, impurities such as calcium and magnesium ions often replace iron ions in the crystal lattice in isomorphous forms, resulting in an average iron grade of around 33% in actual mining. Furthermore, siderite itself is weakly magnetic, making it difficult to achieve ideal separation parameters using conventional physical beneficiation methods (such as gravity separation, magnetic separation, and flotation). Therefore, roasting-magnetic separation has become the main technical route to improve the utilization efficiency of siderite. Its core lies in using high-temperature roasting to induce a mineral phase transformation in siderite, generating strongly magnetic magnetite or maghemite, followed by efficient recovery through magnetic separation.

[0003] From a thermodynamic perspective, siderite first undergoes a decomposition reaction under a high-temperature neutral atmosphere: FeCO3 → FeO + CO2. The resulting ferrous oxide (FeO) is a weakly magnetic mineral and cannot be effectively recovered directly by magnetic separation. Therefore, it must be further converted into strongly magnetic magnetite (Fe3O4). Theoretically, FeO can be converted into magnetite by reacting with CO2 (3FeO + CO2 → Fe3O4 + CO) or with O2 (3FeO + 0.5O2 → Fe3O4). However, in actual roasting processes, precise control of these reactions presents significant challenges. On the one hand, if the system is in a completely neutral or weakly reducing atmosphere, siderite decomposes and often generates a large amount of ferrous oxide intermediate phase, resulting in insufficient magnetite formation and weak overall magnetism of the roasted product. On the other hand, if an oxidizing atmosphere is introduced after decomposition to promote further transformation of FeO, Fe3O4 has higher reactivity under an oxidizing atmosphere and is very likely to continue to oxidize to form hematite (2Fe3O4+0.5O2→3Fe2O3). Hematite is a weakly magnetic mineral, and once formed, it will seriously degrade the selectivity of the roasted product.

[0004] To address the aforementioned issues, researchers have attempted various technological improvements. Some literature reports the use of a pre-oxidation-magnetization roasting process, where siderite first forms a porous structure before reduction roasting to enhance the magnetization effect. Other studies have proposed introducing appropriate amounts of reducing gases (such as CO and H2) during roasting to suppress excessive FeO oxidation. However, these methods largely rely on continuous control of the external atmosphere, requiring high equipment and operational precision, and still struggle to avoid Fe3O4 over-oxidation due to atmosphere fluctuations or improper reaction time control. In particular, existing technologies lack effective targeted treatment methods for the FeO mesophase remaining after neutral roasting of siderite: without treatment, the FeO residue rate is high, limiting magnetic separation recovery; if air is directly introduced for continuous oxidation, the reaction is highly susceptible to exceeding the Fe3O4 stage and entering the hematite region due to excessively high oxygen potential or prolonged reaction time. Even with continuous ventilation under low oxygen concentration conditions, the risk of peroxidation still exists because Fe3O4 and FeO coexist in the same reaction system, making it difficult for oxygen to selectively act on FeO while remaining inert to Fe3O4.

[0005] In conclusion, developing a roasting method that can accurately convert FeO into magnetite is of great significance for improving the comprehensive utilization level of siderite resources. Summary of the Invention

[0006] The purpose of this invention is to provide a method for the precise conversion of siderite into magnetite through neutral roasting. This method accurately converts the ferrous oxide mesophase generated during neutral roasting into magnetite, while effectively avoiding the over-oxidation of magnetite to form hematite, significantly improving the magnetic properties of the roasting product, and creating conditions for efficient recovery through subsequent magnetic separation.

[0007] To achieve the above objectives, the present invention provides the following technical solution: A method for the precise conversion of siderite to magnetite by neutral roasting includes the following steps: (1) Place the siderite-containing ore sample in a roasting apparatus, introduce nitrogen gas, roast, and obtain an intermediate product containing ferrous oxide. (2) Keep the temperature constant, stop the nitrogen gas supply, close the outlet valve of the calcination device for 1 to 2 minutes and then open it, keep it open for 30 to 60 seconds, and repeat the closing-opening operation 3 to 5 times; (3) Purge with nitrogen gas; (4) Keep the temperature constant and introduce a mixture of nitrogen and air into the device. After the gas is introduced, immediately close the air passage and only introduce nitrogen. Repeat this process 1 to 3 times to obtain a roasted product with magnetite as the main iron phase.

[0008] Furthermore, the roasting temperature in step (1) is 600–750°C.

[0009] Furthermore, the roasting time in step (1) is 20 to 40 minutes.

[0010] Furthermore, during the closing-opening operation described in step (2), the peak pressure inside the device is maintained at 0.05 to 0.1 MPa during the valve closing period by adjusting the closing time.

[0011] Further, in step (3), nitrogen gas is introduced to purge for 3-5 minutes.

[0012] Furthermore, in step (4), the flow rate of the mixed gas is 200-500 mL / min.

[0013] Furthermore, in step (4), the volume fraction of air in the mixed gas is controlled to be 1% to 4%.

[0014] Furthermore, the time for a single pass of mixed gas in step (4) is 20 to 40 seconds.

[0015] Furthermore, in step (4), a nitrogen atmosphere is maintained for 3 to 5 minutes between two adjacent gas mixing cycles.

[0016] Furthermore, the number of times the mixed gas process is passed in step (4) is determined based on the ratio of ferrous oxide to total iron in the intermediate product sampled and detected after step (3): when FeO / TFe≥0.6, the mixed gas process is passed 3 times; when 0.5≤FeO / TFe<0.6, the mixed gas process is passed 2 times; when FeO / TFe<0.5, the mixed gas process is passed once.

[0017] In step (1) of this invention, the siderite is neutrally roasted under a nitrogen atmosphere. The siderite undergoes a thermal decomposition reaction, generating ferrous oxide and releasing carbon dioxide gas. Since nitrogen is continuously flowing as a carrier gas, the carbon dioxide generated by the decomposition is carried away in time, allowing the decomposition reaction to continue in the positive direction, ultimately obtaining an intermediate product with ferrous oxide as the main phase. At the same time, a small amount of incompletely decomposed siderite cores may remain in the material. In step (2), when the nitrogen supply is stopped and the outlet valve is closed, a closed system is formed inside the device. At this time, the carbon dioxide released by the continued decomposition of the remaining siderite cannot be discharged, causing the pressure inside the device to gradually increase. As the pressure rises, carbon dioxide gas is forced into the surface pores and internal microcracks of the ferrous oxide particles. When the valve is opened, the pressure inside the device drops instantly to atmospheric pressure. The high-pressure gas forced into the particles expands rapidly and rushes outward. This process carries out the gas and fine particles in the pores, and at the same time, it impacts the pore walls, causing the original pores to expand or connect, and even generating new microcracks along grain boundaries or defects inside the particles. After several shut-off-open cycles, the porosity and specific surface area of ​​the ferrous oxide particles increased significantly, forming gas diffusion channels that are conducive to subsequent gas-solid reactions. At the same time, a small amount of ferrous oxide underwent a self-magnetization reaction, promoting the formation of magnetite.

[0018] Step (4) of the staged mixed gas operation is the key control link for the conversion of ferrous oxide to magnetite. Because the ferrous oxide treated in step (2) has a well-developed porous structure, oxygen can quickly diffuse into the interior of the particles and contact the ferrous oxide. The volume fraction of air in the introduced gas is controlled at a low level, which limits the total amount of oxygen entering the system per unit time; the single gas introduction time is strictly limited to a short period, so that the oxidation reaction is interrupted before reaching thermodynamic equilibrium. The oxygen source for continued oxidation is immediately cut off after each gas introduction, thus controlling the reaction product at the magnetite stage. A nitrogen atmosphere is maintained between adjacent gas introductions, allowing the magnetite layer generated in the previous reaction to achieve structural stability under oxygen-free conditions, while simultaneously ensuring uniform heat distribution within the system, avoiding excessively high local temperatures that could lead to runaway reactions. After several intermittent and limited oxidation treatments, ferrous oxide gradually transforms into magnetite, while the magnetite, due to timely interruption of each oxidation, does not have the opportunity to continue oxidizing to form hematite. The final roasted product is dominated by magnetite, with significantly enhanced magnetism, meeting the requirements for subsequent magnetic separation and recovery.

[0019] Compared with the prior art, the advantages and beneficial effects of the present invention are as follows: 1. Utilizing self-generated gas to activate materials without introducing impurities: In step (2) of this invention, CO2 released from the decomposition of siderite is used for valve pulse opening and closing operations. Through pressure oscillation, microcracks and interconnected pores are generated inside the ferrous oxide particles, increasing the specific surface area and gas diffusion channels. The entire process does not require the addition of any external substances, avoiding the impact of impurities on the quality of subsequent products.

[0020] 2. Precise micro-oxygen control in stages to avoid over-oxidation: Step (4) of this invention adopts a staged ventilation method with low oxygen concentration and short duration. After each ventilation, the nitrogen atmosphere is immediately restored, strictly controlling the oxidation reaction within the magnetite formation stage. Compared with continuous ventilation, staged ventilation effectively prevents magnetite from further oxidizing into hematite, ensuring the high magnetic susceptibility of the product.

[0021] 3. Dynamic adjustment based on the state of intermediate products to achieve intelligent control: This invention dynamically determines the number of ventilations in step (4) based on the FeO / TFe ratio of the intermediate products sampled and detected after step (3). This on-demand oxygen supply strategy ensures sufficient conversion while avoiding over-processing, embodying the core concept of precise conversion.

[0022] 4. Minimal equipment modification and easy industrialization: This invention only requires adding valve control function to the existing fluidized bed roasting device, without the need for complex equipment modification. The process is simple to operate and easy to achieve industrial application. Attached Figure Description

[0023] Figure 1 This is a schematic diagram of the process for the precise conversion of siderite into magnetite by neutral roasting according to the present invention.

[0024] Figure 2 The images show X-ray diffraction patterns of the calcined products obtained in Examples 1-3 and Comparative Examples 1-5. Detailed Implementation

[0025] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0026] Example 1 This embodiment provides a method for the precise conversion of siderite to magnetite through neutral roasting, such as... Figure 1 As shown, it includes the following steps: (1) A siderite ore sample with a particle size of -0.5 mm was selected as raw material after crushing and screening. The sample was chemically analyzed and the results were TFe 43.6%, FeO 52.61%, and loss on ignition 36.46%. 200g of the sample was placed in a laboratory fluidized bed roasting apparatus, and nitrogen gas was introduced at a flow rate of 200mL / min as the fluidizing gas. The sample was heated to 700℃ at a heating rate of 10℃ / min and roasted at 700℃ for 30min to obtain an intermediate product containing ferrous oxide.

[0027] (2) Maintain the calcination temperature at 700℃, stop the nitrogen supply, close the outlet valve of the calcination device, and record the closing time. After closing for 1.5 minutes, quickly open the valve and maintain the open state for 45 seconds to allow the pressure inside the device to return to normal. Repeat the above closing-opening operation 4 times. During the operation, monitor the pressure inside the device using a pressure sensor. The peak pressure inside the device during valve closure is 0.06~0.08MPa.

[0028] (3) Nitrogen gas was introduced into the calcination apparatus at a flow rate of 300 mL / min and purged for 5 min to remove residual carbon dioxide gas. After purging, a small amount of intermediate product sample was taken from the apparatus for chemical analysis to determine its FeO content and TFe content. The FeO / TFe ratio was calculated to be 0.48.

[0029] (4) Keeping the calcination temperature constant at 700℃, a mixture of nitrogen and air is introduced into the apparatus at a total gas flow rate of 300 mL / min, controlling the volume fraction of air in the mixture to be 2%, and the duration of each gas introduction is 30 s. Immediately after the gas introduction is completed, the air passage is closed and the nitrogen atmosphere is restored. The above gas introduction process is performed only once, and then the apparatus is naturally cooled to room temperature under a nitrogen atmosphere to obtain the calcined product.

[0030] Chemical analysis of the calcined product obtained in this embodiment showed that TFe was 68.42%, FeO was 28.71%, and FeO / TFe ratio was 0.45. Figure 2 X-ray diffraction analysis showed that the roasted product was mainly composed of magnetite, with extremely weak characteristic peaks of ferrous oxide. This indicates that the method in this embodiment efficiently converted ferrous oxide into magnetite without over-oxidation.

[0031] Example 2 This embodiment provides a method for the precise conversion of siderite to magnetite through neutral roasting, including the following steps: (1) Take the same siderite ore as in Example 1, and select a sample with a particle size of -0.5 mm after crushing and screening. Place 200 g of the sample in a laboratory fluidized bed roasting apparatus, introduce nitrogen gas as the fluidizing gas at a flow rate of 200 mL / min, heat to 680 °C at a heating rate of 10 °C / min, and roast at 680 °C for 35 min to obtain an intermediate product containing ferrous oxide.

[0032] (2) Maintain the calcination temperature at 680℃, stop the nitrogen supply, close the outlet valve of the calcination device, close it for 2 minutes, then open the valve and keep it open for 30 seconds to allow the pressure inside the device to return to normal. Repeat the above closing-opening operation 5 times. During the operation, monitor the pressure inside the device using a pressure sensor. The peak pressure inside the device during valve closure is 0.07 to 0.09 MPa.

[0033] (3) Nitrogen gas was introduced into the calcination apparatus at a flow rate of 300 mL / min and purged for 4 min to remove residual carbon dioxide gas. After purging, a small amount of intermediate product sample was taken from the apparatus for chemical analysis to determine its FeO content and TFe content, and the FeO / TFe ratio was calculated to be 0.55.

[0034] (4) Keep the calcination temperature constant at 680℃, and introduce a mixture of air and nitrogen into the apparatus at a total gas flow rate of 400 mL / min, controlling the air volume fraction in the mixture to be 3%, with each gas introduction time being 25 s. Immediately after each gas introduction, close the air channel and resume nitrogen introduction, maintaining a nitrogen atmosphere for 4 min between adjacent gas introductions. After completing two gas introduction processes, allow the mixture to cool naturally to room temperature under a nitrogen atmosphere to obtain the calcined product.

[0035] Chemical analysis of the calcined product obtained in this embodiment showed that TFe was 67.93%, FeO was 29.89%, and FeO / TFe was 0.44. Figure 2 X-ray diffraction analysis showed that the roasted product was mainly composed of magnetite, with weak characteristic peaks of ferrous oxide. This indicates that the method in this embodiment efficiently converted ferrous oxide into magnetite without over-oxidation.

[0036] Example 3 This embodiment provides a method for the precise conversion of siderite to magnetite through neutral roasting, including the following steps: (1) Take the same siderite ore as in Example 1, and select a sample with a particle size of -0.5 mm after crushing and screening. Place 200 g of the sample in a laboratory fluidized bed roasting apparatus, introduce nitrogen gas as the fluidizing gas at a flow rate of 200 mL / min, heat to 720 °C at a heating rate of 10 °C / min, and roast at 720 °C for 25 min to obtain an intermediate product containing ferrous oxide.

[0037] (2) Keep the calcination temperature constant at 720℃, stop the nitrogen supply, close the outlet valve of the calcination device, close the valve for 1 minute, open the valve, and keep it open for 60 seconds to allow the pressure inside the device to return to normal. Repeat the above closing-opening operation 3 times. During the operation, monitor the pressure inside the device through a pressure sensor. The peak pressure inside the device during the valve closure period is 0.05~0.07MPa.

[0038] (3) Nitrogen gas was introduced into the calcination apparatus at a flow rate of 300 mL / min and purged for 5 min to remove residual carbon dioxide gas. After purging, a small amount of intermediate product sample was taken from the apparatus for chemical analysis to determine its FeO content and TFe content. The FeO / TFe ratio was calculated to be 0.63.

[0039] (4) Keep the calcination temperature constant at 720℃, and introduce a mixture of air and nitrogen into the apparatus at a total gas flow rate of 250 mL / min. Control the volume fraction of air in the mixture to be 1.5%, and the duration of each gas introduction is 35 s. Immediately after each gas introduction, close the air passage and resume nitrogen introduction. Maintain a nitrogen atmosphere for 3 min between adjacent gas introductions. After completing three gas introduction processes, allow the mixture to cool naturally to room temperature under a nitrogen atmosphere to obtain the calcined product.

[0040] Chemical analysis of the calcined product obtained in this embodiment showed that TFe was 67.21%, FeO was 30.24%, and FeO / TFe was 0.45. Figure 2 X-ray diffraction analysis showed that the roasted product was mainly composed of magnetite, with weak characteristic peaks of ferrous oxide. This indicates that the method in this embodiment efficiently converted ferrous oxide into magnetite without over-oxidation.

[0041] Comparative Example 1 The difference between this comparative example and Example 1 is that step (4) is: keeping the calcination temperature constant at 700°C, continuously introducing a mixture of air and nitrogen for 5 minutes at a total gas flow rate of 300 mL / min, with an air volume fraction of 2%, and then naturally cooling to room temperature under a nitrogen atmosphere to obtain the calcined product.

[0042] Chemical analysis of the calcined product obtained in this comparative example showed that TFe was 63.20%, FeO was 25.91%, and FeO / TFe was 0.41. Figure 2 X-ray diffraction analysis showed that obvious hematite characteristic peaks appeared in the roasted product. This indicates that even with a low oxygen volume fraction, continuous aeration still resulted in some magnetite being further oxidized to hematite, and the product showed a tendency for peroxidation.

[0043] Comparative Example 2 The difference between this comparative example and Example 2 is that step (4) is: keeping the calcination temperature constant at 680°C, continuously introducing a mixture of air and nitrogen for 5 minutes at a total gas flow rate of 400 mL / min, with an air volume fraction of 3%, and then naturally cooling to room temperature under a nitrogen atmosphere to obtain the calcined product.

[0044] Chemical analysis of the calcined product obtained in this comparative example showed that TFe was 61.10%, FeO was 22.00%, and FeO / TFe was 0.36. Figure 2 X-ray diffraction analysis showed that the characteristic peaks of hematite in the roasted products were significantly enhanced, indicating that it had become one of the main phases. This suggests that when continuous aeration with a higher oxygen volume fraction is used, the peroxidation problem is more severe, and a large amount of magnetite is oxidized to hematite.

[0045] Comparative Example 3 The difference between this comparative example and Example 3 is as follows: Step (4) is: keeping the calcination temperature constant at 720℃, a mixture of air and nitrogen is introduced into the device at a total gas flow rate of 250 mL / min, controlling the air volume fraction to be 1.5%, and the single gas introduction time to be 35 s. After the gas introduction is completed, the air channel is immediately closed and nitrogen is introduced again. The above mixed gas introduction process is performed only once, and then the device is naturally cooled to room temperature under a nitrogen atmosphere to obtain the calcined product.

[0046] Chemical analysis of the calcined product obtained in this comparative example showed that TFe was 67.45%, FeO was 34.40%, and FeO / TFe was 0.51. Figure 2 X-ray diffraction analysis showed that, in addition to magnetite, the roasted product still contained obvious characteristic peaks of ferrous oxide. This indicates that when the FeO / TFe ratio of the intermediate product is high, a single aeration process is insufficient to fully convert ferrous oxide into magnetite, leaving a large amount of unconverted ferrous oxide in the product.

[0047] Comparative Example 4 This comparative example provides a method for the precise conversion of siderite to magnetite by neutral roasting, including the following steps: (1) Take the same siderite ore as in Example 1, and select a sample with a particle size of -0.5 mm after crushing and screening. Place 200 g of the sample in a laboratory fluidized bed roasting apparatus, introduce nitrogen gas as the fluidizing gas at a flow rate of 200 mL / min, heat to 700 °C at a heating rate of 10 °C / min, and roast at 700 °C for 30 min to obtain an intermediate product containing ferrous oxide.

[0048] (2) Nitrogen gas was introduced into the calcination device at a flow rate of 300 mL / min and purged for 5 min. After purging, a small amount of intermediate product sample was taken from the device for chemical analysis to determine its FeO content and TFe content. The FeO / TFe ratio was calculated to be 0.66.

[0049] (3) Keep the calcination temperature constant at 700℃, and introduce a mixture of air and nitrogen into the apparatus at a total gas flow rate of 300 mL / min, controlling the air volume fraction to be 2%, and the single gas introduction time to be 30 s. Immediately after each gas introduction, close the air channel and resume nitrogen introduction, and maintain a nitrogen atmosphere for 4 min between adjacent gas introductions. After completing 3 gas introduction processes, allow the apparatus to cool naturally to room temperature under a nitrogen atmosphere to obtain the calcined product.

[0050] Chemical analysis of the calcined product obtained in this comparative example showed that TFe was 66.82%, FeO was 34.75%, and FeO / TFe was 0.52. Figure 2 X-ray diffraction analysis showed that magnetite and ferrous oxide coexisted in the roasted product, with ferrous oxide having a distinct characteristic peak.

[0051] Comparative Example 5 The difference between this comparative example and Example 1 is as follows: Step (2) is: keep the calcination temperature constant at 700℃, stop the nitrogen supply, close the outlet valve of the calcination device, open the valve after 30 seconds, and maintain the open state for 3 minutes to allow the pressure inside the device to return to normal pressure. The above closing-opening operation is repeated 4 times. During the operation, the pressure inside the device is monitored by a pressure sensor. The peak pressure inside the device during the valve closure period is only 0.01~0.02MPa.

[0052] Chemical analysis of the calcined product obtained in this comparative example showed that TFe was 67.12%, FeO was 32.22%, and FeO / TFe was 0.48. Figure 2 X-ray diffraction analysis showed that magnetite was the main phase in the roasted product, but there were still obvious characteristic peaks of ferrous oxide.

[0053] The results show that the present invention physically activates ferrous oxide particles through the valve pulse opening and closing operation in step (2), significantly improving the efficiency of subsequent oxidation reactions. Through the staged micro-oxygen operation in step (4) and precise control of the aeration frequency based on the FeO / TFe ratio of the intermediate product, the efficient conversion of ferrous oxide to magnetite is achieved, while effectively avoiding the over-oxidation of magnetite to form hematite. The final FeO / TFe ratio of Examples 1-3 is consistently within the ideal range of 0.44 to 0.45, and XRD shows that magnetite is the main phase. Comparative Examples 1-2 show that continuous aeration easily leads to over-oxidation; Comparative Example 3 shows that insufficient aeration frequency results in incomplete conversion; Comparative Example 4 shows that even increasing the aeration frequency is difficult to achieve the ideal effect when the pulse activation in step (2) is lacking; and Comparative Example 5 shows that the activation effect decreases significantly when the parameters in step (2) exceed the range of the present invention. In summary, the steps of the present invention work together to achieve the precise conversion of siderite to magnetite.

[0054] The above description represents the preferred embodiments of the present invention. It should be noted that those skilled in the art can make various improvements and modifications without departing from the principles of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for the precise conversion of siderite to magnetite through neutral roasting, characterized in that, Includes the following steps: (1) Place the siderite-containing ore sample in a roasting apparatus, introduce nitrogen gas, roast, and obtain an intermediate product containing ferrous oxide. (2) Keep the temperature constant, stop the nitrogen gas supply, close the outlet valve of the calcination device for 1 to 2 minutes and then open it, keep it open for 30 to 60 seconds, and repeat the closing-opening operation 3 to 5 times; (3) Purge with nitrogen gas; (4) Keep the temperature constant and introduce a mixture of nitrogen and air into the device. After the gas is introduced, immediately close the air passage and only introduce nitrogen. Repeat this process 1 to 3 times to obtain a roasted product with magnetite as the main iron phase.

2. The method for precisely converting siderite into magnetite by neutral roasting according to claim 1, characterized in that, The roasting temperature in step (1) is 600-750℃.

3. The method for precisely converting siderite into magnetite by neutral roasting according to claim 1, characterized in that, The roasting time in step (1) is 20 to 40 minutes.

4. The method for precisely converting siderite into magnetite by neutral roasting according to claim 1, characterized in that, During the closing-opening operation described in step (2), the peak pressure inside the device is maintained at 0.05 to 0.1 MPa during the valve closing period by adjusting the closing time.

5. The method for precisely converting siderite into magnetite by neutral roasting according to claim 1, characterized in that, Step (3) Purge with nitrogen gas for 3-5 minutes.

6. The method for precisely converting siderite into magnetite by neutral roasting according to claim 1, characterized in that, The flow rate of the mixed gas introduced in step (4) is 200-500 mL / min.

7. The method for precisely converting siderite into magnetite by neutral roasting according to claim 1, characterized in that, In step (4), the volume fraction of air in the mixed gas is controlled to be 1% to 4%.

8. The method for precisely converting siderite into magnetite by neutral roasting according to claim 1, characterized in that, In step (4), the time for a single pass of mixed gas is 20 to 40 seconds.

9. The method for precisely converting siderite into magnetite by neutral roasting according to claim 1, characterized in that, In step (4), maintain a nitrogen atmosphere for 3 to 5 minutes between two consecutive gas mixing cycles.

10. The method for precisely converting siderite into magnetite by neutral roasting according to claim 1, characterized in that, The number of times the mixed gas is passed in step (4) is determined based on the ratio of ferrous oxide to total iron in the intermediate product sampled and detected after step (3): when FeO / TFe≥0.6, the mixed gas is passed 3 times; when 0.5≤FeO / TFe<0.6, the mixed gas is passed 2 times; when FeO / TFe<0.5, the mixed gas is passed once.

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